Sifting through new Census data, I came upon a file containing data on fertility rates by several characteristics of the mother. Although whites, blacks, and Asians have all become less fecund over the last thirty years [or not--please see Hail's comment below], their declines have been almost exactly offset by a rapid increase in the size of the more procreative Hispanic population, such that the total fertility rate in the US today is right where it was in 1980.
Also of interest are shifts in fertility patterns by age that have occurred over the last three decades. The following table shows the percentage changes in birth rates by the mother's age from 1980 to 2008:
Women are having children later than they did a generation ago. The bulk of all birthing is still done by women in their 20s (see below), so the large percentage changes among women in their teens and thirties is less impactful on the whole that it might appear at first glance.
Educational romanticism and the accompanying societal desire that everybody receive higher education means more and more women are delaying childbirth or foregoing it entirely. The consequences are not only demographic, they are also health-related, as several risk factors for the child increase alongside the age of the delivering mother.
Now for the percentage of total births by age range of the mother in 1980 and 2008. Data are not available for women 40+ in the earlier period, so I estimated using simple algebra to get the total for the 40-44 and 45-54 age ranges, and then split that number proportionally in accordance to the most recent year in which births for women in those age ranges are recorded:
A plurality of births are now to women aged 25-29, a change from three decades ago, when women aged 20-24 gave birth most frequently. In 1980, women in their 20s accounted for 64.6%--nearly two-thirds--of all live births, while women in their 30s accounted for 19.1%, or less than one-fifth of the total. Today, at 53.0% of all births, women in their 20s account for just over half. Women in their 30s now account for 34.0%--over one-third. If these trends continue, in another generation it will be more common for babies to be born to mothers in their 30s than to mothers in their 20s.